12-8-09 - Biology 330 Class Notes We learn something about...

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Biology 330 – Class Notes – 12/8/09 We learn something about the source of the material being consumed – what we learn from food webs Detritus – overwhelming significance in the movement of material through the food web Most of the food chains are only a couple of links (herbivores to primary producers) 3. Connectance – how well connected these food webs are. * For example: Figure 23.7 * The maximum number of connections (links) = [(s)*(s-1)]/2 maximum number of links Connectance (C) = (Number of links) / (number of links possible) S = number of species In this example: [(13)*(12)]/2 = 78 Food web is undersaturated when the value of connectance is less than 1. WAY, WAY Undersaturated food webs is the RULE what we see most of the time. o This means the number of actual links is LESS than the number of possible chains. Food web theorists computer modelists They have been trying to determine what the properties of food webs are that promote stability STABILITY : How resistant is the community is to change (perturbations) How resilient the community is to change (how quickly does diversity return to the level before the perturbation) o Example: if we have a forest fire, early fire, how resistant is the community to that perturbation and how long does it take to return to normal after the perturbation takes place. Food web theorists simulations What attributes of the food webs PREVIOUSLY THOUGHT (before food web theorists ran their calculations/model): Food webs that have LOTS of species with LOTS of connectance MOST STABLE o Reason: if there are a lot of connections, then it won’t matter as much if one species dies o They are more resistant to change because you can remove some subset of species and it wont matter because there are enough connections between species to maintain stability
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o THIS IS WRONG Food web theorists: Their equation to model interactions o (average interaction strength)*(number of species*connectance)^(1/2) < 1 You don’t have to know this IS THERE ANY DATA TO SUPPORT THIS? YES!!!!!!!!!!!!!!!!
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12-8-09 - Biology 330 Class Notes We learn something about...

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